Semiconductor devices are used in a large number of electronic devices, such as computers, cell phones, and others. Semiconductor devices comprise integrated circuits that are formed on semiconductor wafers by depositing many types of thin films of material over the semiconductor wafers, and patterning the thin films of material to form the integrated circuits. Integrated circuits include field-effect transistors (FETs) such as metal oxide semiconductor (MOS) transistors.
One of the goals of the semiconductor industry is to continue shrinking the size and increasing the speed of individual FETs. To achieve these goals, fin FETs (FinFETs) or multiple gate transistors are used in sub 32 nm transistor nodes. FinFETs not only improve areal density, but also improve gate control of the channel.
In order to set the threshold voltage (Vt) for a FinFET, a work function (WF) metal is included in the gate stack. Because the gate stack of the FinFET is formed over both the top and sidewalls of the channel, a conformal process is needed to form the gate stack. Unfortunately, the physical vapor deposition (PVD) process typically used to form the N work function metal (e.g., titanium aluminide, TiAl) for a planar device is not suitable for formation of the gate stack in the FinFET. Indeed, the physical vapor deposition is not conformal.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.